It is best to treat in a small QT container using premixed treatment water so that you have more control and ease of observation.
For tattered fins-often the environment is the cause and by QT'ing this can help rule that out-if he doesn't have fin rot-just fresh, dechlorinated, like temp water and water changes is sometimes the best treatment.

Why are you needing to add salt-generally if you need to treat the Betta it is best to remove them to QT and treat with premixed treatment water.
Some tetras can be sensitive to salt in therapeutic doses needed to treat a Betta

I've never heard of people using earthworms; I doubt they'd be very nutritious. Also if you catch them yourselves there is the risk of having pesticides absorbed from the grass and dirt they've been exposed to.
And haha, thanks! There is still SO much to learn though! I learn so much more everyday! :P

Sorry for the late reply! I don't come on here much! I go on the leo forum more! ;)
You can feed mealworms or superworms. I don't recommend superworms until the leo is about 6 months old, because they are quite big. Mealworms, are quite small and a leo can eat about 10-15 in a sitting. Superworms, are larger and leos eat max usually 3 or 4 in a sitting, every other day.
You can also feed crickets. Dubai roaches are also an AWESOME feeder (very nutrituous and easy to keep), but they are banned in some countries/states. For example, they are banned in Canada and California, but I don't know about anywhere besides that.
Worms like waxworms, phoenix worms and butterworms are good for treats maximum once a week, but once a month is better. Variety is the best. I usually switch between superworms, mealworms and crickets every few days.